This invention relates to novel acyclic chiral derivatives of hibiscus acid and the process of preparing the same.
Hibiscus acid, [(+)-Hydroxycitric acid lactone or (2S,3R)-Tetrahydro-3-hydroxy-5-oxo-2,3-furandicarboxylic acid], can be isolated from the leaves/fruit calyxes of Hibiscus sabdariffa or from the leaves of Hibiscus furcatus, and Hibiscus cannabinus. However the non-availability of Hibiscus acid in the market, in the optically pure form, has resulted in the limited use of Hibiscus acid or its derivatives in the broad area of organic synthesis and pharmaceutical front. In U.S. patent application Ser. No. 09/365,300, the large scale manufacture of Hibiscus acid in the optically pure crystalline form has been described.
During the past two decades there has been a great deal of interest in finding cheap and potential chiral derivatives from chiral pool to accomplish synthetic pathways with a high degree of asymmetric induction.
The object of the present invention is to synthesize novel acyclic chiral derivatives of Hibiscus acid which are found to be important building blocks in organic synthesis and are extensively used for the preparation of optically active ligands and biologically active products.
To achieve the said objective this invention provides a novel acyclic chiral compound of Hibiscus acid of formula I, 
wherein:
R1=R5=lower aryl or alkyl ester or substituted aryl or alkyl alcohol
R3=substituted aryl or alkyl ester or substituted aryl alcohol
R2=R4=hydroxyl or 
In the above formula I:
R1 and R5 is selected from xe2x80x94COOCH3, xe2x80x94COOC2H5, xe2x80x94COOCH(CH3)2, xe2x80x94C(Ph)2OH, xe2x80x94C(4-MePh)2OH, xe2x80x94C(1-Naphth)2OH
R2 and R4 is OH or 
R3 is selected from xe2x80x94CH2COOCH3, xe2x80x94CH2COOC2H5, xe2x80x94CH2COOCH(CH3)2, xe2x80x94CH2C(Ph)2OH, xe2x80x94CH2C(4-MePh)2OH, xe2x80x94CH2C(1-Naphth)2OH
to form various chiral derivatives, namely, chiral triesters, chiral ketals, chiral alcohols.
Chiral Triester Derivatives
Iaxe2x80x94R1=R5=xe2x80x94COOCH3, R2=R4=xe2x80x94OH and R3=xe2x80x94CH2COOCH3 
Ibxe2x80x94R1=R5=xe2x80x94COOC2H5, R2=R4=xe2x80x94OH R3=xe2x80x94CH2COOC2H5 
Icxe2x80x94R1=R5=xe2x80x94COOCH(CH3)2, R2=R432 xe2x80x94OH and R3=xe2x80x94CH2COOCH(CH3)2 
Chiral Ketal Derivatives
Idxe2x80x94
xe2x80x83R1=R5=xe2x80x94COOCH3, R3=xe2x80x94CH2COOCH3 
Iexe2x80x94
xe2x80x83R1=R5=xe2x80x94COOC2H5 and R3=xe2x80x94CH2COOC2H5 
Chiral Alcohol Derivatives(diols)
Ifxe2x80x94
xe2x80x83R1=R5=xe2x80x94C(Ph)2OH and R3=xe2x80x94CH2CPh)2OH
Igxe2x80x94
xe2x80x83R1=R5=xe2x80x94C(4-MePh)2OH and R3=xe2x80x94CH2C(4-MePh)2OH
Ihxe2x80x94
xe2x80x83R1=R5=xe2x80x94C(1-Naphth)2OH and R3=xe2x80x94CH2C(1-Naphth)2OH
Compound of formula Ia is trimethyl(1S,2R)-1,2-dihydroxy-1,2,3-propanetricarboxylate
Compound of formula Ib is Triethyl(1S,2R)-1,2-dihydroxy-1,2,3-propanetricarboxylate.
Compound of formula Ic is Tri isopropyl(1S,2R)-1,2-dihydroxy-1,2,3-propanetricarboxylate.
Compound of formula Id is dimethyl(4S,5R)-2,2-dimethyl-4-(2-oxo-2-methoxyethyl)-1,3-dioxolane-4,5-dicarboxylate (Id)
Compound of formula Ie is diethyl(4S,5R)-2,2-dimethyl-4-(2-oxo-2-ethoxyethyl)-1,3-dioxolane-4,5-dicarboxylate (Ie)
Compound of formula If is (4S,5R)-4-(2-hydroxy-2,2-diphenylethyl)-2,2-dimethyl-alpha,alpha,alphaxe2x80x2,alphaxe2x80x2-tetraphenyl-1,3-dioxolane-4,5-dimethanol (If).
The present invention further includes a process for preparing the acyclic chiral triester of formulae Ia-Ic comprising
refluxing hibiscus acid with appropriate alcohol in presence of an inorganic catalyst for 6-12 hours,
adjusting the pH of the reaction mixture to neutral using aqueous alkali solution,
concentrating the said reaction-mixture by evaporation,
extracting the said concentrate with an organic solvent,
concentrating the said extract to yield the said product.
The said appropriate alcohols are selected from methanol, ethanol and isopropanol.
The said catalyst is conc. HCl and said organic solvent is chloroform.
The present invention also includes a process for preparing the acyclic chiral triester of formulae If-Ih comprising:
adding solution of chiral acetal/ketal in an organic solvent to a solution of appropriate grignard reagent(ArMgX) in an organic solvent,
refluxing the mixture for 10-20 hours,
adding the inorganic salt solution to the chilled reaction mixture,
collecting the organic phase and extracting the aqueous layer further with a suitable organic acid,
drying the organic extract using a suitable salt,
evaporating the said extract,
subjecting the residue to chromatography.
The organic solvent is tetra hydro furan (THF).
The said appropriate grignard reagent is phenyl Mg bromide, methylphenyl Mg bromide, naphthyl Mg bromide.
The said inorganic salt is ammonium chloride.
The said organic solvent used for extraction is ether.
The said salt used for drying the extract is sodium sulphate.
The chromatography employed for purification is column chromatography. The gel used for chromatography is silica gel. The eluant used for chromatography is hexane chloroform mixture.
Summary of the chiral derivatives of Hibiscus acid is given below in scheme I:
1*/8 
Ia: R1=R5=xe2x80x94COOCH3; R2=R4=xe2x80x94OH; R3=xe2x80x94CH2COOCH3 
Ib: R1=R3=xe2x80x94COOC2H5; R2=R=xe2x80x94OH; R3=xe2x80x94CH2COOCH5 
Ic: R1=R5=xe2x80x94COOCH(CH3)2; R2=R4=xe2x80x94OH; R3=xe2x80x94CH2COOCH(CH3)2 
Id: 
xe2x80x83R1=R5=xe2x80x94COOCH3; R3=xe2x80x94CH2COOCH3 
Ie: 
xe2x80x83R1=R5=xe2x80x94COOC2H5; R3=xe2x80x94CH2COOC2H5 
If: 
xe2x80x83R1R5=xe2x80x94C(Ph)2OH; R3=xe2x80x94CH2C(Ph)2OH
Ig: 
xe2x80x83R1=R5=xe2x80x94C(4MePh)2OH; R3=xe2x80x94CH2C(4-MePh)2OH
Ih: 
xe2x80x83R1=R5=xe2x80x94C(1-Naphth)2OH; R3=xe2x80x94CH2C(1-Naphth)2OH
Standard procedures are followed for the preparation of various chiral ketals of the type Id and Ie.
The invention will now be described with reference to the foregoing examples: